Allow User To Select Row And Populate Textbox With Columns Details

Basically, I have a textbox where a user can enter an ID from a database. To make things easier, the user can popup a GridView using a ModalPopupExtender to display all the tables columns, and they can Select a row, which then closes the modalpopupextender and sets the textbox to the row's ID column.

So far I have this:

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
string testing = GridView1.SelectedRow.Cells[1].Text.ToString();
((TextBox)dtlsInsert.FindControl("txtNom")).Text = GridView1.SelectedRow.Cells[1].Text.ToString();
}

The 'testing' string gets populated, but it doesn't seem to populate the txtNom textbox. Also, how do I close the modalpopupextender programatically?